1. Field of the Invention
The present invention relates generally to measuring the polarization dependent loss (PDL) of an optical device, and particularly to an apparatus and method that substantially eliminates the PDL noise associated with the measurement system such that a component PDL can be more accurately measured.
2. Technical Background
In a single mode fiber, the fundamental mode of the fiber is the solution to the wave equation that satisfies the boundary conditions at the core-cladding interface. There are two solutions to the wave equation that correspond to the fundamental mode, both of which have the same propagation constant. These two solutions are referred to as the polarization modes. The electric field associated with the fundamental mode is assumed to be a transverse field, with the polarization components being linearly polarized along mutually orthogonal x and y directions. As light propagates in a fiber or in a component, the energy of the light is divided between the two polarization modes. The state of polarization refers to the distribution of light energy between these two modes. A device or component may exhibit loss as a function of the polarization mode. The difference in the loss between the two polarization modes represents the polarization dependent loss (PDL) of the device. The PDL of an optical component is a critical parameter that is often used to characterize an optical device being incorporated into an optical fiber network. As optical systems become more complex, system designers are requiring that components and devices meet stricter PDL tolerances because of the cumulative nature of PDL. A device having a PDL of 0.01 dB can have a significant impact in a system that consists of many devices and spans a considerable distance. To meet this need, measurement systems must be able to accurately measure the PDL of such devices.
In one approach that has been considered, a 2xc3x972 3 dB coupler is used to make the PDL measurement of a reflective optical device. At one end, a first port is connected to a polarization controller and a laser light source and a second port is connected to a power detector. At the other end, a third port is connected to the device under test (DUT) and the fourth port is terminated. The laser light and polarization controller inject a randomly polarized light signal into the coupler. The light signal is directed out of the third port and into the DUT, where it is reflected back into the third port. After propagating in the coupler, the reflected signal is directed out of the second port and into the detector which detects the PDL. This system has a major drawback. The coupler itself has a PDL in the range between 0.05 db and 0.1 dB. Since the PDL of the coupler is higher than the PDL of many of the devices now being deployed in photonic systems, the measurement apparatus is unable to measure the true PDL of the device under test because it is hidden within the PDL noise of the system.
In another approach that has been considered, an automated system is provided to measure the PDL of optical devices operating in the forward transmission mode. A light source and a polarization controller are used to provide a signal having four predetermined and unique polarization states. The microprocessor based system causes an actuator to cycle the polarization controller through the four polarization states while a detector is positioned at the output of the DUT to read the intensity of each state. These values are used by a processor to compute the values for each element in a Meuller matrix and to compute a Stokes vector to represent the polarized input signal. These values are then used to compute the PDL of the DUT. The test set has a residual PDL of less than 0.001 dB, which is an improvement. Automated systems can be beneficial, provided that the savings generated by the functionality of the system are more than the attendant software, reliability, and maintainability costs which can be considerable. For example, automated systems are software intensive and it is often the case that the cost of software development exceeds the cost of the hardware. Herein lies the drawback of this system. For a sophisticated system that is so highly automated, it lacks versatility since it is unable to measure devices that work in the backward reflection mode, such as gratings. This in turn leads to an unpleasant choice: purchase or develop a second system for measuring the PDL of reflective devices, or redesign and modify the system in order to accomodate reflective devices. Both solutions involve added expense.
Thus, a need exists for a reliable and relatively inexpensive apparatus for accurately measuring the PDL of an optical device. At the same time, the PDL measurement apparatus must be versatile, having the ability to measure the PDLs of both reflective devices and those that function in the forward transmission mode.
The present invention for an apparatus and method for measuring polarization dependent loss features several fiber optic couplers combined in tandem and oriented such that the PDL noise of the measurement system is reduced to a negligible level. By matching the PDLs of the couplers and vectorally subtracting opposite phases of polarization, the PDL of the measurement system is virtually eliminated. The simplicity of this concept is compelling because it provides a measurement system that accurately measures the PDL of optical components yet in an inexpensive way. It is also a versatile system. It measures the PDL of optical devices that operate in a reflection mode or in a forward transmission mode.
One aspect of the present invention is an apparatus for measuring a polarization dependent loss of an optical device under test using a randomly polarized light signal, the apparatus generates a test input signal that is directed into the optical device under test. The apparatus comprises: a first passive optical element connected to the light source, wherein the first passive optical element has a first polarization dependent loss; and a second passive optical element connected to the first passive optical element and having a second polarization dependent loss that is substantially equal to the first polarization dependent loss. The second passive optical element is disposed relative to the first passive optical element such that the second polarization dependent loss substantially cancels the first polarization dependent loss and outputs the test input signal with a polarization dependent loss equal to a first minimum value.
In another aspect, the present invention includes a method for measuring a polarization dependent loss of an optical device using an apparatus that includes a light source for emitting a light signal that is randomly polarized, a first passive optical element connected to the light source, wherein the first passive optical element has a first polarization dependent loss. The method for measuring includes the steps of: providing a second passive optical element having a second polarization dependent loss that is substantially equal to the first polarization dependent loss and connected to the first passive optical element at a relative position such that the second polarization dependent loss substantially cancels the first polarization dependent loss; directing the light signal into the first passive optical component such that the second passive optical component produces a test input signal, wherein the test input signal has a polarization dependent loss substantially equal to a first minimum value; directing the test input signal into the optical device; and measuring an output signal of the optical device to thereby determine the polarization dependent loss.
In another aspect, the present invention includes a method for calibrating an apparatus used for measuring a polarization dependent loss of an optical device, the apparatus includes a light source for emitting a light signal that is randomly polarized and a first passive optical element having a first polarization dependent loss. The method for calibrating comprising the steps of: providing a second passive optical element having a second polarization dependent loss that is substantially equal to the first polarization dependent loss and connected to the first passive optical element; directing the light signal into the first passive optical component to thereby create. a calibration signal that exits the second passive optical component; and rotating the second passive optical component until a first calibration polarization dependent loss of the calibration signal equals a first minimum value.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.